P>Toxic cyanobacterial blooms are increasing in prevalence. Microcystins are the most commonly produced cyanotoxin. Despite extensive research the variables regulating microcystin production remain unclear. Using a RT-QPCR assay that allowed the precise measurement of mcyE transcriptional gene expression and an ELISA that enabled small changes in total microcystin concentrations to be monitored, we demonstrate for the first time that microcystin production is not always constitutive and that significant up- and downregulation in microcystin synthesis can occur on time scales of 2-6 h. Samples were collected over 3 days from a small eutrophic lake during a dense microcystin-producing Microcystis bloom. McyE gene transcripts were detected in only four out of 14 samples. Vicissitudes in both microcystin quotas and extracellular microcystin levels corresponded with changes in mcyE expression. During the period of exalted microcystin synthesis Microcystis sp. cell concentrations increased from 70 000 cells ml-1 to 4 000 000 cells ml-1. These data provide compelling evidence that changes in Microcystis cell concentrations influence microcystin production.